13-05-2011, 06:52 PM
1. INTRODUCTION
Floods occur when the water draining from a watershed, whether from rainfall and/or melting snow, exceeds the holding capacity of the river or stream channel. During floods, water overflows onto the adjacent low-lying lands called flood plains. In hilly and mountainous areas flooding is likely to be rapid and dangerous. In relatively flat flood plains and low-lying areas, flood water may remain for days or even weeks. In coastal areas flooding are mainly caused by the storm surges associated with tropical storms, hurricanes, tsunamis, etc. Anthropogenic intervention often leads to flood damage. Cumulatively, floods have been our nation’s greatest disaster, disrupting lives, and often causing significant economic losses.
Indiscriminate urbanization has increased the intensity of flood hazard due to the improper construction of pavements, buildings, roads, other impervious structures and choking of drainage. This accelerates the process of run off than infiltration. While flooding in the upstream portion is caused by intense rainfall of short duration, the downstream floods are produced by storms of long duration over a large area. Important factors that have a direct bearing on flood are land use on the higher reaches of the watershed, reclamation of flood plain, lowland, paddy field and siltation of man-made as well as natural drains.
Pazhavangadi Thodu (drain) is one of the important storm water drains, which run through the heart of the Trivandrum city, Kerala. Pazhavangadi Thodu starts from Kerala Water Authority (KWA) at Vellayambalam and emerges into Aamayizhanjan Thodu near Kannamoola and has a length of about 5.75 km. It is the main discharge channel of Trivandrum City and traverses through the key areas like Thampanoor, Pazhavangadi, Thakaraparambu, Vanchiyoor, Pattoor etc.
Recurrent floods as a result of insufficient drainage are the perpetual problems faced by the city over the past two decades or so. Flooding occurs frequently in Thampanoor and East Fort areas. The water collected during a down-pour in the area adjacent to the railway station and bus stand is not drained quickly into the Thodu as the carrying capacity of the leading drains is totally blocked in some places and not sufficient elsewhere. Runoff stagnates for a period of time causing flooding in the Thampanoor area. The most severe and frequent flooding in the city occurs at Thampanoor and East Fort / Pazhavangadi areas, followed by Uppilamoodu and Kannamoola areas.
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NEED FOR STUDY
Recurrent floods as a result of insufficient drainage are the perpetual problems faced by the city over the past two decades or so. Flooding occurs frequently in Thampanoor and East Fort areas. The Pazhavangadi Thodu (natural drain) starting from Kerala Water Authority (KWA) carries the surface water of the upstream reaches and passes through Thampanoor crosses under the railway line and reaches East Fort area. The water collected during a down-pour in the area adjacent to the railway station and bus stand is not drained quickly into the Thodu. When the runoff stagnates for a period of time, it causes flooding in the Thampanoor area. So also the cross-sectional area of the long culvert passing through the railway yard is getting reduced year after year due to the accumulation of debris and silt inside it. Moreover portions of the old culvert structure is said to be still trapped inside it, blocking the already reduced vent-away.
The most severe and frequent flooding in the city occurs at Thampanoor and East Fort / Pazhavangadi areas , followed by Uppilamoodu and Kannamoola areas. Thampanoor is the city’s Central Railway Station and the main KSRTC bus stand area, with very busy vehicular and pedestrian traffic, whole East Fort is one of the main bus stops and the centre of commercial and business activities as well as a number of famous temples including the famous Padmanabha Swami Temple.
The flooding results mainly from two reasons, one, and the existing roadside drains are inadequate, clogged with silt and debris and ill-maintained. Secondly, the receiving stream, namely, the Pazhavangadi and the downstream Aamayizhanjan Thodu are also silted up and backs up into the roadside drains rather than act as a receiving stream. Other causes are inadequate size of culverts and bridges, especially the railway culverts at Thampanoor and Uppilamoodu and water supply and sewer lines crossing the Thodu.
As mentioned earlier the main problem areas that were identified from past experience are Thampanoor and East fort. Once the receiving stream (Pazhavangadi – Aamayizhanjan) is improved, the local solution to the Thampanoor flooding is relatively simple. The problem in Thampanoor area was mainly because the runoff was diverted mostly to Pazhavangadi Thodu opposite the Ganapathy Temple, but rather than receiving the runoff, water from the Thodu backs up into the area and disturbs normal life.
1.1. AIM:
To study the causes of flooding in low lying areas of Trivandrum city during monsoon on the basis of the case study done in the east fort area.
1.2. OBJECTIVES:
• To locate the flooded area in eastfort.
• To identify the causes of flood.
• To suggest appropriate solutions to overcome the flooding.
1.3. METHODOLOGY:
• Study of toposheets to obtain data of natural streams and drains.
• Determine the study area
• Collect rainfall data from KSUDP
• Collect land use map from Land Use Board
• Collect the list of buildings in East Fort area
• Collect the details of drainage network in Trivandrum city from City corporation
• Calculate the catchment area from the contour map.
• Find the total covered area of catchment from aerial photo using digital planimeter.
• Study the capacity of drain and check whether it is sufficient or not.
• Check the maintenance of drains.
• By the use of ArcGIS software determine the percentage of built up area
• Check whether rain water harvesting is done.
• Suggest methods to reduce run off to low lying areas.
• Suggest methods to reduce the floods in low lying areas.
2. LITERATURE REVIEW
2.1. FLOODS IN THIRUVANANTHAPURAM
The city is bounded with hills, valleys, Karamana river and streams that debouch into the Akkulam-Veli lake. The general slope of the city is from east to west. The northern and eastern areas of the City Corporation is bounded by Pulayanar Kotta hills,Thirumala hills, Mudavanmugal Hills etc. the two rivers (Karamana and its tributary Killi) and the drainage thodu (Pattom thodu, Ulloor thodu) flow through the valleys in between the hillsand empties in Akkulam kayal. The relief of the city area varies from 0 to 80 metres above PMSL. Rain water getting collected in the city, finds its way to Lakshadweep sea through Ulloor thodu, Pattom thodu and Vanchiyoor thodu join just upstream of Kannammoola bridgeand form Amayizhanchan thodu before reaching Arabian Sea through Akkulam Kayal. These thodus also carry the runoff of Thiruvananthapuram city apart from the Karamana river.
With increase in population and high demand of land, the wetlands in the city are being filled up for public housing and industrial purposes. For example the present Putharikandam Maidanam and Bakery Junction (where Reserve bank building is situated) were once marshy land. Because of the increasing tendency of filling the low lying marshy areas, the flood absorbing capacity of Thiruvananthapuram city has considerably reduced.
Flood in the Thiruvananthapuram city is accentuated mainly due to the flood in the Karamana river. Various human interventions badly affect the ecological balance of the river, which eventually has a great impact on floods. The Hindu daily (dated 3rd Oct, 2003) reported illegal extraction of sand from the Upper reaches of the Karamana river. Extensive sand mining, not only deplete the ground water table but also affects the stability of river banks, thus leading to the loss of whole chunks of land and making large areas flood-prone.
2.2. RAINWATER HARVESTING SYSTEM
In most urban areas, population is increasing rapidly and the issue of supplying adequate water to meet societal needs and to ensure equity in access to water is one of the most urgent and significant challenges faced by decision-makers. With respect to the physical alternatives to fulfil sustainable management of freshwater, there are two solutions: finding alternate or additional water resources using conventional centralised approaches; or better utilising the limited amount of water resources available in a more efficient way. To date, much attention has been given to the first option and only limited attention has been given to optimising water management systems. Among the various alternative technologies to augment freshwater resources, rainwater harvesting and utilisation is a decentralised, environmentally sound solution, which can avoid many environmental problems often caused in conventional large-scale projects using centralised approaches.
ADVANTAGES OF RAINWATER HARVESTING METHOD
Rainwater harvesting systems can provide water at or near the point where water is needed or used. The systems can be both owner and utility operated and managed. Rainwater collected using existing structures (i.e., rooftops, parking lots, playgrounds, parks, ponds, flood plains, etc.), has few negative environmental impacts compared to other technologies for water resources development. Rainwater is relatively clean and the quality is usually acceptable for many purposes with little or even no treatment. The physical and chemical properties of rainwater are usually superior to sources of groundwater that may have been subjected to contamination.
Other advantages are:
• Rainwater harvesting can co‐exist with and provide a good supplement to other water sources and utility systems, thus relieving pressure on other water sources.
• Rainwater harvesting provides a water supply buffer for use in times of emergency or breakdown of the public water supply systems, particularly during natural disasters.
• Rainwater harvesting can reduce storm drainage load and flooding in city streets.
• Users of rainwater are usually the owners who operate and manage the catchment system, hence, they are more likely to exercise water conservation because they know how much water is in storage and they will try to prevent the storage tank from drying up.
• Rainwater harvesting technologies are flexible and can be built to meet almost any
• requirements. Construction, operation, and maintenance are not labour intensive
3. DATA COLLECTION
Following data collection works were done for the project work:
• Rainfall intensity (mm/hr) details for different durations and return periods of Trivandrum city were obtained from Kerala Sustainable Urban Development Project for the purpose of calculating runoff.
• Drain details of Pazhavangadi thodu were collected from the Irrigation Department in order to determine the slope and bed levels at different sections of the drain.
• The number of buildings that were built at different zones of the district, over the time period of 2005 – 2011, was obtained from Trivandrum City Corporation
• The land use map of Karamana basin (2007), was obtained from the Land use board.
4. DRAINAGE SYSTEM IN THE CITY AREA
The city has large network of storm water canals and drains. However, most of the storm water drains and canals in the city carry semi treated and untreated sewage and have been depositories for solid waste. Though the natural topography of the city in general has adequate slope to drain out the storm water, the poor maintenance of the major drainage outlets of the city and the frequent blockage of certain important drains within the city are mainly responsible for local flooding and water logging. Water logging (localized flooding) occurs in a few areas of the city resulting in inconvenience to the city dwellers. Normally, water logging takes place during the period of high intensity rainfall and / or for extended duration.
The drainage network of the city consists of two major rivers, a few canals (Thodus), their feeders and lakes. The two major rivers are Karamana and Killi which flow through the city area. There are a large number of natural drains (Thodu) and leading drains and a large network of roadside drains in the city area like Pazhavangadi, Ulloor, Pattom, Kannamoola (Aamayizhanjan) etc. The booming development and construction activities have resulted in large scale filling of natural drains, wetlands and ponds and the consequent blockage of drains and reduction in groundwater recharge. Recurrent floods in some busy parts of the city are extremely frequent and disrupt normal life with loss of work, business and even property as also creating health hazard, for which solutions are to be found.
PAZHAVANGADI THODU
Pazhavangadi Thodu starts from KWA water treatment plants, Vellayambalam and merges into Aamayizhanjan Thodu near Kannamoola and has a length of about 5.75 km. It is the main discharge channel of Trivandrum City and traverses through the key areas like Thampanoor, Pazhavangadi, Thakaraparambu, Vanchiyoor, Pattoor etc. The width of the Thodu varies from 3 to 6 m. Many road drains discharge into this Thodu. The road culverts were noticed to be the main hindrance for the free flow of discharge through the Thodu and have been re-constructed to ensure free flow. The re-construction of the culvert and desilting of the Thodu has decreased the incidence of flooding to a certain extent. But periodic desilting is essential in this portion since it passes through the heart of the city covering Bakery junction, KSRTC Bus stand, Railway station, Vanchiyoor, Pattoor etc. The Pazhavangadi Thodu joins the Aamayizhanjan Thodu stream from the left at about 0.25 km below the Ulloor – Pattom Thodu confluence.
The roadside storm water drains in the area that outfall into the Pazhavangadi Thodu often backs up and floods the Thampanoor, Mele Pazhavangadi and East Fort areas and disturbs the busy pedestrian and vehicular traffic. The Thodu occasionally overflows its banks as it comes out of the culvert in the Mele Pazhavangadi area in front of the Ganapathy temple. It is unfortunate that the indiscriminate human abuse of this Thodu has turned it into the main cause of flooding in these and other areas, rather than being a boon to relieve the excessive runoff resulting from the ever-continuing urbanization.
